Large intestine — gotta stay hydrated

large intestine gotta stay hydrated

In a cast of organs that so far includes the pancreas, heart, pharynx, and assorted smaller beings, this week we’ll look at the large intestine.

First,  let’s set the record straight:  The only thing large about this intestine is its width — it needs a spacious interior, to accommodate the slowly desiccating remains of your meal.  By far the longest part of your intestines is the small intestine — the site where the great preponderance of digestion and nutrient absorption occur in the body.

What’s passed on to the large intestine is a soggy slurry of undigestible food bits — especially the undigestible long-chain carbohydrates known as “fiber”.  What remains of value in this mix is water and electrolytes (such as sodium and potassium), which the large intestine absorbs.

Don’t be misled, though — the large intestine is vital for your survival.  All the organs of the digestive system from the mouth to the small intestine secrete large amounts of fluid.  These secretions add digestive enzymes and other additives to process the food.  Enough fluid is released in this way, that you’d quickly dehydrate, without the large intestine’s help.  Indeed, the inability to absorb fluids in the large intestine, resulting in watery feces, or diarrhea, is a deadly condition that kills millions every year.

Incidentally, the mouth of our thirsty friend is accurately placed — it represents the opening where the small intestine attaches, and thus, releases its slushy contents into the large intestine.  At that junction, an ileocecal valve prevents backflow of feces into the small intestine.  Other parts are (more or less) anatomically correct as well — from the portly cecum (shown as the “body” of our absorptive acquaintance), and a tail-like vermiform appendix, through the ascending colon, transverse colon, descending colon, sigmoid colon (the S-shaped “zigzag” near the end), rectum, and anal canal.  The end!

The versatile pharynx

Pharynx depicted with legs, eyes, and nose.

Here’s a body part you’ve probably never seen in isolation — and almost certainly not with legs, and a face of its own!  But these embellishments serve to emphasize that the pharynx is a middle-man, straddling two different organ systems and negotiating the competing demands of each.

Ideally, air travels in through the nasal cavity, all three parts of the pharynx (nasopharynx, oropharynx, laryngopharynx), and then the larynx, on its way to the lungs.  While eating, breathing is paused while food travels through the oral cavity (mouth), the lower parts of the pharynx (oropharynx, laryngopharynx), and the esophagus on its way to the stomach.  But as you can see, these two pathways overlap in the pharynx.  As a result, the pharynx, along with associated structures, plays a vital role in regulating the passage of air and food.

There are four ways that food and air movement can differ from the ideal situation depicted above, and these “mistakes” vary all the way from innocuous (and perhaps entertaining) to life-threatening.

First, food could move through the upper pharynx (nasopharynx).  Sometimes food is expelled this way if you sneeze or laugh while food is in your mouth.  But most of the time, the soft palate, a muscular structure, closes against the back of the pharynx to prevent this.  If you want to “get in touch” with your soft palate, try snorting – you just closed your soft palate and then opened it while inhaling.  A word to the wise: Restrict your snorting to air and mucus!  But unfortunately, if you can think of it, somebody will try it — the FDA had to issue a warning not to snort chocolate powder.

Second, air can move between the middle pharynx (oropharynx) and the mouth.  Wait, you may say, isn’t this exactly how we breathe?  If that’s your impression, then you probably have a congested nasal cavity, and it’s true we can breathe through the mouth, although it’s less than ideal.  The reason is that the nasal cavity has complex folds that help to trap debris and pathogens and do an optimal job of “conditioning” the air before it reaches the lungs.  So if, as I often do, you find yourself walking behind a group of sneezing and coughing students, it may help to close your mouth and breathe through your nose.

Like the rest of your respiratory and digestive tracts, the pharynx is a moist habitat suitable for invading microbes, and being near the entrance to both tracts, it is a common place for infections to begin.  What’s known as a “sore throat” is generally a viral infection of the pharynx.  That’s likely why we have several immune structures — the tonsils — in the walls of the pharynx.

Third, air can move between the lower pharynx (laryngopharynx) and the esophagus – the food tube leading to the stomach.  According to healthline, in most of us about two quarts of air move into the stomach this way, each day, from the small amounts of air trapped in our food or drink.  And of course, when air leaves the body through this route it is called burping — technically known as eructation.

The fourth, and by far the most serious problem, is food traveling from the laryngopharynx into the larynx.  The larynx, or voicebox, sits in front of the esophagus and is the gateway for the lower respiratory tract.  “Inhaling” your food is dangerous — in the US, over 5,000 people die from choking each year. Our main protection from choking is the epiglottis — a laryngeal cartilage between the oropharynx and nasopharynx, which folds down to close off the entrance to the larynx, every time you swallow.

Your pharynx handles a lot of traffic through the body, and deserves some respect – definitely nothing to snort at!


Pancreas adds the special sauce

Last time we looked at a gland in the stomach where acid and an enzyme are secreted to begin the breakdown of protein.  I talked about the chyme that is produced by the mixing of stomach secretions with the food. As chyme leaves the stomach (symbolized by the hamburger — although in reality it is now mush), it enters the duodenum (shown here as a pink tube) — the first part of the small intestine, and by far the shortest  (the original term “duodenum digitorum” meant it was “twelve fingers’ widths” in length).  It’s a tiny part of the 10-foot long small intestine, but a vital part, because this is where several key ingredients are added.

The pancreas is far more important for digestion than the stomach because it secretes enzymes to digest all three food groups — fats, carbohydrates, and proteins.  It’s a somewhat fish-shaped gland, that sits behind the stomach.  These enzymes don’t function well in an acid environment, so the pancreas throws in a little natural “alka-seltzer” (bicarbonate) to neutralize the acid, and all this is poured together into the duodenum, through a duct.

The word pancreas means “all flesh” because there is little in the pancreas other than soft, secretory cells (nothing tough, no gristle).  I tried it once in New Orleans (it’s called “sweetbread” on the dinner table), and it wasn’t bad at all.

A gutsy first post

This view of a gastric gland marks the first post in this blog about biological cartoons.  The goal of the blog is to share some of the cartoons I’ve been using in my classes, and meet up with like-minded folks who share my view — that most lectures and nonfiction books are better off with a few cartoons. I will also broaden the discussion at times to look at the whole genre of educational cartooning, and share with you some of the best examples I’ve found.

But my training is in biology, so that will be my main focus.  The choice of the gastric gland is perfect, not just because I will be lecturing on it tomorrow (really!), but it also represents the anxiety (and a little extra stomach acid) I have in launching this project. I have never done a blog before; it sounded like too much time commitment.  But as the cartooning has become central among my interests, I realized it was time to move beyond the powerpoint slides and reach out — I’m hoping at the very least, there are a handful of teachers out there who might find a place for these cartoons in their classroom.

Since space is not an issue here, what I will do is show the cartoons unlabeled, and provide explanatory text, with the main terms in bold.  The drawing shows the main cells of the stomach epithelium (orange) secreting mucus, which protects the epithelium from its own secretions. Nevertheless, there’s rapid turnover of cells (replaced every few days), and a couple hapless oldsters are shown plunging into the roiling cauldron of stomach acid and food, known as chyme. This fortunate term (which, happily, comes from the same Indo-European root as “humor”!) gives rise to a plethora of puns, one of which is posted on the “wall” next to the “police chief” cells representing chief cells.  These cells secrete pepsinogen, an enzyme precursor, which when activated (by acid) digests proteins in the stomach. So the police chief is truly “taking a bite out of chyme”, which is a reference to the “take a bite out of crime” ad campaign from the 80’s.

Stomach acid is secreted by parietal cells (blue), which are the most realistic drawings in the figure — their “teeth” are the many microvilli (convolutions of the membrane) that add surface area, maximizing the ability to secrete acid. At the bottom of the gland is an enteroendocrine cell (purple), a general term for the many epithelial cells of your gut that secrete hormones.  The cell is “tasting” the food molecules in the gland, and sending out a hormone into the bloodstream to activate other glands of the stomach. Finally, the pink cells with pacifiers are (you guessed it) baby cells, or stem cells, which have a sort of “immortality”, continuing to divide throughout your life, while their progeny migrate up and down to become the replacement cells in the epithelium.